Include a new set_state attribute to sim.config

[deepanshs]

All calculations in mrsimulator assume solid-state NMR spectrum simulation averaging over thoughts of orientations over multiple sidebands. For liquid state NMR simulation, only isotropic contributions are relevant, and therefore, computing frequencies over multiple orientations and sidebands is not optimum.

Proposed solution
Add a new sim.config.set_state attribute, an enumeration with two literals [solid, liquid]

• When sim.config.set_state="solid" (default), the code is as is.

• When sim.config.set_state="liquid", the following changes must be forces.

sim.config.integration_density=1,
sim.config.integration_volume="octant",
sim.config.number_of_sidebands=1,
sim.config.number_of_gamma_angles=1

The default freq_contrib for liquids is ["Shielding1_0", "J1_0"]